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Leaching behavior of microplastics during sludge mechanical dewatering and its effect on activated sludge
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Researchers studied how mechanical dewatering of sewage sludge affects the microplastics embedded within it, including their physical changes and leaching behavior. They found that the dewatering process roughened microplastic surfaces, reduced particle sizes, and altered their chemical properties due to dewatering agents and mechanical forces. The study also showed that leachates from these processed microplastics could negatively affect activated sludge performance in treatment systems.
Dewatering is an indispensable link in sludge treatment, but its effect on the microplastics (MPs) remains inadequately understood. This study investigated the physicochemical changes and leaching behavior of MPs during the mechanical dewatering of sludge, as well as the impact of MP leachates on activated sludge (AS). After sludge dewatering, MPs exhibit rougher surfaces, decreased sizes and altered functional groups due to the addition of dewatering agents and the application of mechanical force. Meanwhile, plastic additives, depolymerization products, and derivatives of their interactions are leached from MPs during sludge dewatering process. The concentration of MP-based leachates in sludge is 2-25 times higher than that in water. The enhancement of pH and ionic strength caused by dewatering agents induces the release of MP leachates enriched with protein-like, fulvic acid-like, and soluble microbial by-product-like substances. The reflux of MP leachates in sludge dewatering liquor to the wastewater treatment system negatively impacts AS, leading to a decrease in COD removal rate and inhibition of the extracellular polymeric substances secretion. More importantly, MP leachates cause oxidative stress to microbial cells and alter the microbial community structure of AS at the phylum and genus levels. These findings confirm that MPs undergo aging and leaching during sludge dewatering process, and MP leachates may negatively affect the wastewater treatment system.
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Researchers studied how 60 days of exposure to different sizes and types of microplastics affected activated sludge at wastewater treatment plants. Millimeter-sized particles dramatically reduced sludge dewaterability by up to 47.7%, mainly through physical disruption of sludge flocs. Nano-sized particles also impaired sludge performance by inhibiting microbial activity and altering the composition of extracellular polymeric substances.
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